Ethylene alpha-olefin (polyethylene) copolymers are typically produced in a low pressure reactor, utilizing, for example, solution, slurry, or gas phase polymerization processes. Polymerization takes place in the presence of catalyst systems such as those employing, for example, a Ziegler-Natta catalyst, a chromium based catalyst, a metallocene catalyst, or combinations thereof.
A number of catalyst compositions containing single site, e.g., metallocene, catalysts have been used to prepare polyethylene copolymers, producing relatively homogeneous copolymers at good polymerization rates. In contrast to traditional Ziegler-Natta catalyst compositions, single site catalyst compositions, such as metallocene catalysts, are catalytic compounds in which each catalyst molecule contains one or only a few polymerization sites. Single site catalysts often produce polyethylene copolymers that have a narrow molecular weight distribution. Although there are single site catalysts that can produce broader molecular weight distributions, these catalysts often show a narrowing of the molecular weight distribution as the reaction temperature is increased, for example, to increase production rates. Further, a single site catalyst will often incorporate comonomer among the molecules of the polyethylene copolymer at a relatively uniform rate. The molecular weight distribution (MWD) and the amount of comonomer incorporation can be used to determine a SCBD.
For an ethylene alpha-olefin copolymer, short chain branching (SCB) on a polymer chain is typically created through comonomer incorporation during polymerization. Short chain branch distribution (SCBD) refers to the distribution of short chain branches within a molecule or among different molecules that comprise the polyethylene polymer. When the amount of SCB varies among the polyethylene molecules, the resin is said to have a “broad” SCBD. When the amount of SCB is similar among the polyethylene molecules of different chain lengths, the SCBD is said to be “narrow”.
SCBD is known to influence the properties of copolymers, for example, stiffness, toughness, extractable content, environmental stress crack resistance, and heat sealing, among other properties. SCBD of a polyolefin may be readily measured by methods known in the art, for example, Temperature Raising Elution Fractionation (TREF) or Crystallization Analysis Fractionation (CRYSTAF).
It is generally known in the art that a polyolefin's MWD and SCBD is largely dictated by the type of catalyst used and is often invariable for a given catalyst system. Ziegler-Natta catalysts and chromium based catalysts produce polymers with broad SCBD, whereas metallocene catalysts normally produce polymers with narrow SCBD. It has been long observed in the industry that there are trade-off paradigms among the different product attributes; most noticeably among stiffness, toughness, and processability (S/T/P). Since the introduction of metallocene in 1990s, some of such paradigms have been relaxed significantly with careful manipulations of molecular structure and composition in the product.
Polymers having a broad orthogonal composition distribution (BOCD) in which the comonomer is incorporated preferentially in the high molecular weight chains can lead to improved physical properties, for example, stiffness, toughness, processability, and environmental stress crack resistance (ESCR), among others. Because of the improved physical properties of polymers with orthogonal composition distributions needed for commercially desirable products, there exists a need for controlled techniques for forming polyethylene copolymers having a broad orthogonal composition distribution.